The invention relates to catheters, and more particularly to ablation catheters capable of creating linear lesions.
It has long been known that the action of the heart depends on electrical signals within the heart tissue. Sometimes these electrical signals become faulty. It has been found that ablating (burning) these cardiac conduction pathways in the region of the problem destroys the tissue to eliminate the faulty signal. Ablation is also used therapeutically with other organ tissue, such as the liver, prostate and uterus.
Electrophysiology (EP) catheters are catheters having one or more electrodes at their tips and are used for both diagnosis and therapy. Electrodes at the tips of EP catheters allow the physician to measure electrical signals along the surface of the heart (called mapping) then, when necessary, ablate certain tissues using, typically, radio frequency (RF) energy conducted to one or more ablation electrodes.
Sometimes ablation is necessary only at discrete positions along the chamber wall. This is the case when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line, called linear ablation. This is the case for atrial fibrillation, where the aim is to reduce the total mass of contiguous (electrically connected) atrial tissue below a threshold believed to be critical for sustaining multiple reentrant wavelets. Linear lesions are created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass.
Linear ablation is typically accomplished in one of several ways. One way is to position the tip portion of the catheter so that an ablation electrode is located at one end of the target site, apply energy to the electrode to ablate the tissue adjacent to the electrode and then slide the tip portion along the chamber wall and repeat the ablation sequence. This is sometimes referred to as the burn-drag-burn technique. This technique is time-consuming (which is not good for the patient) and requires multiple accurate placements of the electrode (which may be difficult for the physician). Another way is to use a catheter having the series of spaced-apart band or coil electrodes which, after the electrode portion of the catheter has been properly positioned, are energized simultaneously or one at a time to create the desired lesion. If the electrodes are close enough together the lesions run together sufficiently to create a continuous linear lesion. While this technique eliminates some of the problems associated with the burn-drag-burn technique, some repositioning of the catheter may be required to create an adequately long lesion. In addition, it may be difficult to obtain adequate tissue contact pressure for each electrode in a multi-electrode catheter. Also, the use of multiple electrodes to create the linear lesion tends to make the tip portion more expensive to make, more bulky and may cause the tip portion to be stiffer than is possible when a single, or very few, electrodes are used. The added complications resulting from the use of multiple ablation electrodes can also reduce overall reliability.
Another ablation-related problem results from the delivery of RF energy to muscular tissue, such as the heart. Ablation of such tissue using conventional ablation catheters has a tendency to char or burn the contacting blood or tissue with which the electrodes are in contact if the temperatures exceed a certain threshold (generally 100.degree. C.). This increases the difficulty of the ablation process by necessitating removal of the catheter to clean the tip portion after a series of burns.